Two-Century-Old Light Trick Creates Swirling ‘Hedgehog’ Skyrmions

In 1818, a French physicist named Augustin-Jean Fresnel predicted a strange thing: if you shine light on a small disk, a bright spot would appear at the center of the shadow. Critics scoffed. Then an experiment proved him right. That spot — now called the Poisson spot — was a key proof that light behaves as a wave. Two centuries later, that same phenomenon is enabling something far stranger: tiny, hedgehog-like swirls of light called optical skyrmions.

A team from Nanyang Technological University, Singapore (NTU Singapore) has revived this classic optical trick to create stable, swirling patterns in the properties of light. The patterns, known as optical skyrmions, resemble the spikes of a hedgehog radiating outward from a center. They are topological structures — configurations that can’t be smoothed away without tearing — and they’ve been a hot topic in physics for years, mostly in magnetic materials. But now, researchers have shown they can be generated in a simple optical setup using a phenomenon first described over 200 years ago.

“We were fascinated by the fact that a 200-year-old phenomenon could be harnessed to create such intricate topological structures,” said Dr. Yijie Shen, assistant professor at NTU’s School of Physical and Mathematical Sciences and lead author of the study. The work was published in Nature Communications and has already sparked interest among physicists working on light manipulation and information encoding.

What Are Optical Skyrmions?

Skyrmions were first theorized in the 1960s by British physicist Tony Skyrme to describe particle-like configurations in nuclear physics. They later found a home in condensed matter physics, where magnetic skyrmions — tiny swirling spin textures — are studied for next-generation data storage. But in optics, skyrmions are all about the polarization of light: the direction in which the electric field oscillates. In an optical skyrmion, the polarization direction rotates smoothly as you move outward from the center, forming a kind of vortex.

Imagine a hedgehog rolling into a ball: the spines point in all directions. In an optical skyrmion, the polarization vectors point outward radially from a central point. This is a Néel-type skyrmion, named after the French physicist Louis Néel. Until now, generating such optical skyrmions required complex setups with multiple lasers or special metasurfaces. The NTU team, however, achieved it with nothing more than a coherent light source, a small opaque disk, and a camera. Simple. And that’s the beauty of it.

“The Poisson spot is essentially the bright core of the shadow of a disk,” explained Prof. Nikolay Zheludev, a senior author on the study and director of NTU’s Centre for Disruptive Photonic Technologies. “We realized that if we carefully control the phase and polarization of the light hitting the disk, the spot itself becomes a skyrmion.”

The Poisson Spot Revival

Poisson spot (also called the spot of Arago) has been a staple in textbooks for decades. It’s the result of Fresnel diffraction: light waves bending around the edges of a disk and constructively interfering at the center. In the NTU experiment, the team didn’t just use any disk. They used a disk coated with a thin metallic film that introduced a specific polarization pattern. When laser light passed through a polarizer and then a quarter-wave plate before hitting the disk, the resulting diffraction pattern at the shadow center exhibited a perfect radial polarization distribution — the hallmark of a skyrmion.

The researchers confirmed the skyrmion’s topological nature by measuring the skyrmion number, a topological charge that counts how many times the polarization winds around. They found a number of 1, meaning it’s a fundamental skyrmion. And unlike many previous demonstrations, this one didn’t require extreme conditions or nanofabrication. It even worked with a simple laser pointer and a pinhole-sized disk. “This is a remarkably simple experiment,” said Dr. Shen. “It shows that complex topological structures can emerge from basic wave physics.”

Interestingly, this isn’t the only recent discovery that mimics biological structures in nature. For instance, researchers recently uncovered a spring-loaded spider trap in Australia designed to catch ants — a different kind of topological wonder from the natural world. And just as biologists are finding new mechanisms in nature, physicists are finding new ways to harness light.

How They Made Them: The Experimental Setup

The NTU team’s setup was deceptively straightforward. A linearly polarized laser beam passed through a quarter-wave plate to become circularly polarized. Then it encountered a small opaque disk (about 1 mm in diameter) mounted on a glass substrate. The disk itself was coated with a thin layer of gold, which introduced a phase shift that varied with the polarization of the incident light. Behind the disk, a camera recorded the intensity pattern of the shadow.

In the center of the shadow, the team observed a bright spot — the Poisson spot. But when they used a polarizing filter in front of the camera, they saw that the spot’s polarization was not uniform; it was radially oriented. The direction of polarization pointed outward from the center like spokes on a wheel. This radial pattern is exactly what defines a skyrmion in the polarization field. Further analysis using Stokes polarimetry confirmed the topological charge.

“We were able to tune the skyrmion size by changing the distance between the disk and the camera,” said Prof. Zheludev. “That gives us a way to create skyrmions of different scales, from micrometers to millimeters.” This scalability is important for potential applications: smaller skyrmions could be used in integrated photonic circuits, while larger ones might be useful for optical trapping or imaging.

The team also demonstrated that the skyrmion could be generated with different colors of light — red, green, and blue — by simply changing the laser wavelength. That’s because the phenomenon relies on diffraction, which works across the visible spectrum. So, in principle, you could create a rainbow of skyrmions. Not bad for a trick that started as a 19th-century proof of wave optics.

What This Means for the Future

So, why should anyone care about a tiny swirl of light in a shadow? Well, topological structures are robust against perturbations. In information technology, that means skyrmions could encode data in a way that resists noise. In optical communications, they could be used to create stable beams that don’t spread out. And in microscopy, specially structured light can improve resolution beyond the diffraction limit.

Understanding these light structures might also help in efforts like bringing back the world’s underwater forests, where precise light delivery could support marine photosynthesis in restoration projects. While that’s a longer-term application, it highlights how fundamental optics can intersect with environmental science.

“This is a beautiful example of how revisiting classic physics can yield new surprises,” said Prof. Zheludev. “We have opened a path to generating topological light structures on demand, using nothing more than a disk and a laser.” The team plans to next explore whether they can create skyrmions with higher topological numbers or even create a lattice of skyrmions using an array of disks. If successful, that could lead to parallel optical data processing.

BBC Future has more on the history of the Poisson spot, and the NTU press release offers additional details. The convergence of two-century-old wave optics and modern topological physics reminds us that sometimes the best new discoveries are hiding in plain sight — right in the shadows.